The present invention relates generally to fuel assemblies and, more particularly, to a detachable connection for attaching a handling socket to the duct tube or housing of a fuel assembly.
The reactor core of a typical nuclear reactor is designed to sustain a continuous sequence or chain of fission reactions, and generally contains a multiplicity of similarly constructed and interchangeable fuel assemblies vertically oriented in a side-by-side relation. Each fuel assembly, in turn, contains a multiplicity of thin, elongated fuel elements or pins containing fissionable material, such as uranium, thorium, and/or plutonium for example. As is known, the heat generated by the fission reactions is transferred to a coolant, such as liquid sodium for example, and subsequently transmitted to a secondary coolant, such as water, for conversion into steam for generating electrical energy.
In addition to the multiplicity of fuel pins, each fuel assembly usually comprises a relatively thin-walled duct tube serving as the jacket or housing for the fuel pins as well as a conduit for the passage of coolant therethrough, a shield/inlet nozzle assembly for introducing the coolant into the fuel assembly, and a handling socket at the other end of the duct tube to facilitate insertion and removal of the assembly into and from the core. Each fuel assembly must maintain its structural integrity during various stages of reactor operation including heat-up, cool-down, shut-down and powered operations as well as withstand the most adverse operative conditions expected during its lifetime.
Each of the fuel elements or pins is comprised of a plurality of uranium or plutonium oxide cylindrical fuel pellets, stacked end-to-end within a thin-walled tube or cladding provided with a spring loaded plenum at one end thereof and sealed at its opposite end with an end cap or plug. The fuel pin cladding is formed of a metallic alloy having good neutron economy, i.e., low neutron cross section and capable of withstanding the adverse effects of thermal cycling, corrosion, irradiation and thermal creep, and irradiation induced changes in material properties.
The duct tubes serve as the axial and lateral fuel assembly support and provide for coolant flow separation between fuel assemblies. As in the case of the fuel pins, the duct tubes also are formed of a special metallic alloy designed to withstand the deleterious effects encountered in a high temperature irradiation environment. The fuel assemblies are vertically oriented in the core and, in order to provide structural continuity therefor, are provided at their lower ends with shield/inlet nozzle assemblies, respectively, and at their upper ends with handling sockets, respectively.
The shield/inlet nozzle assembly allows coolant to enter the fuel assembly while providing neutron shielding. It structurally ties the fuel pins to a support member and normally is welded to the duct tube to provide overall structural integrity for the fuel assembly.
The handling socket allows the coolant to exit the fuel assembly and, like the inlet/nozzle assembly, is normally welded to the duct tube for structural integrity. The handling socket permits manipulation of the fuel asembly and facilitates attachment of grappling devices or hoists thereto for ease of insertion into and removal from the reactor core.
Because of the severe hostile conditions to which fuel assemblies are exposed to in service, they are periodically removed from the core for internal inspection, testing and maintenance, or for fuel pin replacement upon failure thereof. Where prior art fabrication methods employed weldments to attach the handling socket to the duct tube, it can be appreciated that destructive techniques were required to separate the socket from the duct tube with resultant loss of expensive components and materials. Thus, it was found desirable to mechanically attach the socket to the duct tube to realize nondestructive separation of the handling socket from the duct tube for reuse as part of reconstituted fuel assemblies.
Attempts have been made along these lines by utilizing mechanically detachable elements for connecting the handling socket to the duct tube. While these efforts have resulted in attachment assemblies that have admirably served their purpose, they haven't been entirely satisfactory in all cases. For example, one mechanically removable handling socket arrangement utilizes a transition ring that is welded to the duct tube to obtain adequate structural support and remains an integral part thereof. Even though the handling socket can be readily removed for access into the duct tube, the transition ring now forming a permanent part of the duct tube precludes fuel pin removal equipment access to certain fuel pins. Accordingly, failed fuel pins residing in this nonaccessible area cannot be replaced and the otherwise reuseable duct tube must be scrapped. Also, the insertion of the transition ring in the duct tube creates a small crevice that cannot be demonstrated to be completely free of sodium after the cleansing operation. Consequently, total fuel pin reconstitution, which requires a thorough cleansing of all sodium from within the fuel assembly, cannot be performed and weld joint structural integrity could be compromised due to caustic stress corrosion cracking.
Accordingly, it is a primary object of the present invention to overcome the above noted shortcomings by providing an improved connection assembly for releasably attaching a handling socket to a fuel assembly duct tube.
It is another object of this invention to provide a detachable handling socket for a fuel assembly duct tube which can be readily and nondestructively dismantled for reuse and which affords complete and unobstructed access into the interior thereof.
It is a further object of the present invention to provide a novel detachable handling socket-duct tube connection incorporating separable retaining means for securing the components together in an interlocked assembled relation.
It is still a further object of the present invention to provide nondestructive removal of a handling socket from a duct tube to facilitate internal inspection, testing, maintenance, fuel pin replacement and total fuel assembly reconstitution for reuse.